The present invention relates to a single step hydrogenation process for the preparation of p-aminophenol. More particularly, the process relates to the preparation of p-aminophenol in an aqueous acid medium using mono and bimetallic nickel catalysts.
p-Aminophenol (PAP) is a well known and very careful industrial chemical. It is used as an intermediate in the production of pharmaceuticals such as paracetamol, in the production of dyestuffs such as sulphur dyes and in making photographic chemicals.
Conventionally, PAP is prepared by hydrolysing p-nitrochlorobenzene to p-nitrophenol. Hydrogenation of p-nitrophenol to PAP is then carried out using Fe/HCl catalyst. In this multi-step process, quantity of iron (catalyst precursor) required is quite large, subsequently the production of ironxe2x80x94iron oxide sludge is large, posing a serious effluent problem. The work-up of reaction crude is cumbersome. The quantity of iron used is very important for the faster reduction rate.
An important commercial process for the preparation of p-aminophenol involves the catalytic hydrogenation of nitrobenzene in acidic medium using supported platinum based catalysts. In this process phenylhydroxylamine (PHA) is first formed and this intermediate immediately rearranges in the presence of acid to PAP. The major by-product formed is aniline. In actual practice, both these steps are carried out in a single reactor. The reaction mixture consists of both aqueous as well as organic phases.
Reference may be made to U.S. Pat. No. 3,383,416; 1969 by Benner wherein the catalyst reported was Pt/C for hydrogenation of nitrobenzene to p-aminophenol. Greco (U.S. Pat. No. 3,953,509; 1976) reported the use of molybdenum sulphide on carbon catalyst for the hydrogenation of nitrobenzene to PAP. Dunn (U.S. Pat. No. 4,264,529; 1981) has reported the use of platinum on xcex3-alumina for the hydrogenation of nitrobenzene to yield PAP. Low temperature hydrogenation in the presence of modified catalyst system containing sulphur compound and the rearrangement step in a separate vessel has been suggested by Caskey and Chapman (U.S. Pat. No. 4,415,753; 1983). Rylander et al. (U.S. Pat. No. 3,715,397; 1973) disclosed a process for preparation of PAP by catalytic hydrogenation of nitrobenzene in a sulphuric acid medium in the presence of dimethylsulfoxide, using platinum oxide catalyst.
Thus, the catalysts used for hydrogenation of nitrobenzene to p-aminophenol reported in the literature are Pt, Pd, Ru, and PtO2. Among these catalysts, however, Pt is the most active for this system but it is very costly. All these being noble metal the process becomes cost intensive. It also demands to use the same catalyst for several times and also to recover the metal from deactivated catalyst in order to make the process economical.
p-Aminophenol is an important raw material for making paracetamol which is widely used in antipyretic and analgesic drug formulations. Conventional method for preparation of p-aminophenol (PAP) involves reduction of nitrophenol with Fexe2x80x94HCl. This process suffers from major drawbacks such as formation of large amount of sludge (1.2 kg sludge/kg product) posing serious effluent problems and cumbersome work up of reaction crude to obtain pure PAP. Alternate process for PAP involves the catalytic hydrogenation of nitrobenzene using supported platinum catalyst in presence of aqueous acid.
In accordance with this invention, it has now been discovered that group VIII metal like nickel alone or combination of nickel with traces of noble metals like Pt or Pd can be used as an efficient and cheaper catalyst system for hydrogenation of nitrobenzene to PAP. Complete conversion of nitrobenzene is achieved to give PAP as the major product with aniline as a side product.
The main object of the present invention is to provide mono and bimetallic catalysts for single step hydrogenation of nitrobenzene to p-aminophenol, which obviates the drawbacks as detailed above.
The present invention provides a single step process for the preparation of p-aminophenol by hydrogenation of nitrobenzene in presence of an aqueous acid over a mono or bimetallic Ni catalyst, terminating the reaction to obtain a reaction mixture containing the product, extracting the reaction mixture with an organic solvent, separating the aqueous layer containing PAP and neutralising with ammonia solution to separate the solid product.
The present invention provides a single step process for the preparation of p-aminophenol which comprises contacting a mixture of nitrobenzene and aqueous acid with hydrogen over a Ni containing catalyst at a hydrogen pressure upto 1000 psig, at a temperature in the range of 80-120xc2x0 C. for a period of 1 to 4 hrs, terminating the reaction to obtain a reaction mixture containing the product, extracting the reaction mixture with an organic solvent, separating the aqueous layer containing the product, adjusting it""s pH to 7-8 to get solid p-aminophenol.
Accordingly the present invention provides a process for the single step hydrogenation of nitrobenzene to p-aminophneol, said process comprising (a) contacting a mixture of nitrobenzene and aqueous acid with hydrogen mono or bimetallic catalyst at a pressure upto 700 psi, at a temperature 80-120xc2x0 C. for a period of 1-4 hrs, (b) terminating the reaction to obtain product mixture, (c) removing the reaction mixture from the autoclave, (d) separating the catalyst and resin from the reaction mixture by filtration, (e) extracting the filtrate with toluene, (f) analysing the organic and aqueous layers for reactants and products using GC and PLC, (g) treating the aqueous layer with ammonia solution to adjust the pH of solution to 3-4 to partly precipitate PAP, (h) separating the solid thus obtained by filtration, (i) extracting the filtrate with toluene, (j) treating the aqueous layer with ammonia solution to pH 7-8 to substantially precipitate PAP, (k) washing the total solid thus obtained after first and second extraction with distilled water, drying and weighing.
In one embodiment of the invention, the catalyst comprises 10% Ni on a solid support selected from the group consisting of silica, ZSM-5, clay and carbon.
In another embodiment of the invention, a bimetallic catalyst comprising of Ni and traces of one of the group VII metals is used as a catalyst system to increase the selectivity of p-aminophenol.
In yet another embodiment of the present invention, the temperature of the reaction ranges between 80-120xc2x0 C.
In still another embodiment of the present invention, the concentration of acid used is ranges between 2.5-10% w/w.
In another embodiment of the present invention, Pt content of the catalyst is in the range of 0.05-3%.
In another embodiment of the present invention, Ni content of the catalyst is in the range of 5-20%.
In another embodiment the organic solvent used for the extraction of reaction mixture may be selected from toluene, cyclohexane, ethyl acetate of alike.
The following examples are given by way of illustration and therefore should not be construed to limit the scope of present invention.